Reflex space-harmonic oscillator



Jan. 13, 1959 D. A. wATKlNs I2,869,019

REFLEX SPACE-'HARMONIC oscILLAToR Filed Jul;r 7, 1955 United States Patent REFLEX SPACE-HARMONIC OSCILLATOR Dean Allen Watkins, Menlo` Park, Calif., assignorto Hughes'Aircraftl Company, Culver City, Calif., a corporation of Delaware Application July 7, 1955, Serial No. 520,589

Claims. (Cl. S15-3.5)

This invention relates to electron stream microwave oscillators of the reflex type and more particularly to a stream type amplifier tube adapted to provide microwave oscillations capable of being electronically tuned over a broad range of frequencies.

The oscillator of the present invention comprises a space-harmonic slow-wave structure for propagating an electromagnetic Wave at a Velocity substantially less than the velocity of light. One extremity of this slow-wave structure is provided with a reilectionless termination while the other extremity is coupled to an output circuit. An electron gun provides an electron stream which is projected through the' slow-wave structure in a forward direction commencing at the extremity having the reilectionless termination. Upon reaching the opposite extremity of the structure, the electron stream is repelled and displaced slightly in a transverse direction so as not to interfere with the flow of electrons in the forward direction. The repelled electron stream is then directed back towards the electron gun where it is again repelled by the cathode thereof. The electron stream is then directed 'through the structure for a third time after which it is intercepted and collected.

In operation, it is assumed that there is a certain i amount of noise or other perturbationsv of the electrons in the stream. These perturbations of the stream electrons induce corresponding electromagnetic waves on the slowwave structure whence energy from the stream electrons will be transferred to the waves having velocities that are slightly less than that of the electron stream. These amplied waves will, in turn, commence modulating the entire electron stream. Upon reaching the opposite extremity of the slow-Wave structure, the electromagnetic waves are dissipated in the output load and the modulated electron stream repelled and directed back through the slow-wave structure. The modulations of the stream again induce electromagnetic waves on the slow-wave structure only this time they are being propagated towards the electron gun. The waves thus induced on the structure continue to increase the magnitude of the modulations of the repelled electron stream. Upon reaching the electron gun end of the slow-wave structure, the electron stream is again repelled and the energy of the electromagnetic waves dissipated in the reilectionless termination at this end of the structure. The electron stream repelled in this manner constitutes the feedback path of the oscillator of the present invention.

The electron stream thus modulated is then directed through the slow-wave structure in the forward direction for the second time whence the modulations thereof will Linduce corresponding electromagnetic waves on the slowwave structure. In this manner, an electromagnetic wave which has a gain greater than unity upon being propagated forward and back through the slow-wave structure and a phase such that prior modulations on the electron Vstream are reinforced, will increase in magnitude until the entire electron stream is modulated, i. e., the electron Patented dan. i3,

stream has reached saturation. lt is apparent that the frequency of this electromagnetic wave is: determined by the periodicity of the slow-wave structure and the average velocity of the electron stream, Thus it is evident that the frequency of oscillation may be changed by merely changing the velocity of the electron stream through the slow-wave structure.

As is generally known, present day stream-type oscillators may either be of the forward-wave or backward-wave type. In forward-wave stream type oscillators, it is necessary to have either an internal or an external feedback path. ln eithercase, however, the oscillations occur in what is referred to as modes. A mode is present when there is a specific number of wavelengths about the feedback loop. In attempting the electronically tune over a broad band of frequencies, oscillators of this type jump from one mode to the next. Also, it is quite possible for an oscillator of this type to oscillate on several modes at one time which, of course, is undesirable. In the oscillator of the present invention, these disadvantages are eliminated in that a constant number of wavelengths is maintained about the feedback loop, the tuning being entirely dependent on the velocity of the electron stream.

Backward-wave oscillators, on the other hand, may be electronically tuned over a wide range of frequencies but operate at a comparatively low efficiency. ln the present oscillator, electronic tuning comparable to that available with a backward-wave oscillator may be realized together with the higher efficiencies concomitant with forwardwave operation.

It is therefore an object of this invention to provide an electron stream microwave oscillator of the reliex type.

Another object of this invention is to provide a streamtype microwave oscillator capable of being electronically tuned over a continuous wide range of frequencies.

A further object of this invention is to employ a streamtype forward-wave amplifier tube to provide continuous microwave oscillations over a broad band of frequencies.

The novel features which are believed to be characteristic of' the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which an embodiment of the invention is illustrated by way of example. it is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a delinition of the limits of the invention.

Fig. l is a diagrammatic sectional view of an embodiment of the oscillator of the present invention together with associated circuits; and

Fig. 2 is a view taken on section 2 2 of the oscillator of Fig. l. t

Referring now to the drawing, Fig. l shows a cross sectional view of an embodiment of the invention which comprises an electron gun il@ for producing a hollow cylindrical electron stream, a space-harmonic slow-wave structure l2, a solenoid i4 for directing the electron stream lengthwise through the slow-wave structure l2, and a repeller electrode lr6 for reversing the direction of flow of the electron stream afterl having passed through the structure l2.

More particularly, electron gun it) is housed in a glass envelope f8 which is sealed to the left extremity of slowwave structure l?. provided with an aperture 20 to allow passage therethrough of the electron stream. Electron gun 10 comprises a cathode 22 having a circular electron emitting surface 23 which has an aperture through its center portion and is energized by means of a heater 24. A focusing electrode 25 is disposed concentrically about the electron emitting surface 23 at an angle of approximately 671/2u therewith and an accelerating electrode 28 is disposed intermediate the focusing electrode 25 and the aperture 20 in the slow-wave structure 12. In operation, the heater 24 is connected across a battery 26, one terminal of which may be connected to the cathode 22. Cathode 22 together with focusing electrode 26 is connected to ground. Accelerating electrode 2S, on the other hand, is maintained at a potential of the order of 3000 volts positive with respect to ground. This is effected by means of a connection therefrom to the positive terminal of a battery 30, the negative terminal of which is referenced to ground.

The space-harmonic slow-wave structure 12 constitutes a section of rectangular waveguide, as illustrated in cross section in Fig. 2 which has a series of transverse ridges 32 disposed diametrically opposite each other periodically along the inside of the waveguide on the broad sides thereof. The ridges 32 form a corresponding series of transverse slots disposed opposite each other along the path of the electron stream. The width of the slots is made substantially equal to that of the ridges 32. The extremity of slow-wave structure 12 nearest the electron gun 10 forms a 90 bend, whereby a portion 35 of the waveguide is normal to the path of the electron stream. A triangular resistance card 34 is disposed in the center of the portion 35 so as to provide a progressively increasing load on the electric field within this portion of the waveguide thereby effecting a reflectionless termination at this extremity of the slow-wave structure 12. The end of portion 35 of the waveguide is sealed with a shorting member 36 to enable the chamber formed by structure 12 to be evacuated.

At the extremity of slow-wave structure 12 farthest from the electron gun 10, the waveguide is oif-set from the path of the electron stream and an aperture 38 having a predetermined size provided in the wall thereof for the stream. A dielectric member t0 is employed to seal the end of the waveguide at its output extremity to enable it to be evacuated. In operation, the slow-wave structure 12 is maintained at an adjustable potential of the o-rder of 1500 volts positive with respect to ground.

This is accomplished by means of a connection therefrom to a tap 152 of a potentiometer 4liwhich is, in turn, connected across the battery 30.

The solenoid 14 is disposed concentrically about and coextensive with the path of the electron stream. Solenoid 141 is energized with a direct current to produce a magnetic field of the order of from 600 to 1000 gauss along the length of the tube intermediate the electron gun 10 and the repeller electrode 16. This direct current is provided by an adjustable current source 46.

The repeller electrode 16 is disposed within a housing 43 which is sealed externally to the wall of the waveguide about the aperture 38 so as to enable the chamber provided by structure 12 to be evacuated. Repeller electrode 16 has a circular aperture through its central portion to accommodate a longitudinal cylindrical electrode 50 which is disposed therethrough parallel to the path of the electron stream. Electrodes 16, 50 are maintained at adjustable potentials of the order of several hundred volts negative with respect to ground by means of connections to taps 51, 52, respectively, of a potentiometer ft which is, in turn, connected across a battery 56, the positive terminal of which is referenced to ground.

ln the operation of the device of the present invention, the electron stream produced by electron gun is directed through the slow-wave structure 12 and concentrically about the longitudinal cylindrical electrode 50. The electrodes 16, 50 are maintained at potentials negative with respect to the potential of cathode 22 so that the flow of electrons from the electron gun is directed outwards and back through the structure 12. The electrons having been emitted from cathode 22, cannot impinge thereon again. Hence, once again the electrons are repelled and pass .through the structure in the forward direction the second time. The longitudinal cylindrical electrode 50 is made suiciently negative to impart an outward radial component of velocity to the stream electrons to the intent that they are substantially all intercepted by the structure 12 about the aperture 38 when passing hrough the structure for the second time in the forward direction.

The velocity of the stream electrons in passing through the slow-wave structure 12 is determined by the potential impressed thereon relative to the potential of the cathode 22 of electron gun 10. This velocity may be adjusted by positioning the tap 42 of the potentiometer 44. In operation, the velocity of the stream electrons is adjusted so that a given electron travels from opposite one slot to a corresponding position opposite the next successive slot in the time required for its concomitant electromagnetic wave to traverse the same distance along the slow-wave structure 12 plus the time of one cycle, i. e., one period, of the microwave frequency. Under these circumstances the electron stream interacts with a forward space-harmonic component of the wave propagated by the structure 12.

Oscillation occurs when the given electron has made its second trip through the slow-wave structure 12 in the forward direction. The frequency of oscillation is determined by `the voltage impressed on structure 12 relative to that of cathode 22, and the distance from one slot to another through the following relations:

wherein:

Ve is the voltage through which the stream electrons have been accelerated;

uo is the velocity of the stream electrons corresponding to the voltage, Ve;

d is the distance between corresponding points of adjacent slots; and

f is the frequency of oscillation.

The voltage impressed on repeller electrode 16, i. e., the repeller voltage, is adjusted by positioning tap 52 of potentiometer 54 so that at some frequency the transit angle of the electrons is such that they arrive at the end of the structure 12 nearest gun 10 in phase with the wave for their second trip through the slow-wave structure 12. Once this adjustment is made for a particular frequency it will hold for any other frequency selected by an adjustment of the voltage, Ve, that is, the voltage of the structure 12. This is so because the frequency of oscillation is approximtaely proportional to the velocity, uo, and the transit time is inversely proportional to uo. Since the transit angle is the product of frequency and transit time, it is apparent that the transit angle re mains constant for any frequency.

The extremity of slow-wave structure 12 nearest the electron gun 10 is terminated with a reflectionless termination by means o-f the triangular resistance card 34 so that oscillation cannot occur because of the reflection of electromagnetic energy at the end of the waveguide. Also, the wave impedance of the slow-wave structure 12 may be increased by making the width of the waveguide such that the desired frequency of oscillation is close to the low frequency cutoH of the guide. As cutoff is approached, the group velocity of the electromagnetic wave propagated by the slow-wave structure 12 approaches zero with a concomitant increase in impedance presented to the electron stream.

What is claimed is:

1. A microwave oscillator comprising a slow-wave structure having Ia constant periodicity and capable of propagating an electromagnetic wave in both a forward and a backward direction along a path; an electron producing means which also functions to repel electrons disposed at one end of said slow-wave structure for producing an electron stream; means for directing said electron stream at a predetermined velocity along said path through said slow-wave structure in the forward direction, that is, along said path away from said means for producing an electron stream; and means disposed at the rother end of said slow-wave structure for reversing the direction of flow of `said electron stream and for directing it along said path in said backward direction, that is, toward said means for producing an electron stream, at said predetermined Velocity through said slowwave structure to said electron producing means which also functions to repel electrons at said one en-d of said slow-wave structure for reversing the direction of flow of said electron stream again and for directing it along said path at said predetermined velocity through said Islow-wave structure in said forward direction, whereby microwave oscillations having a frequency determined by the periodicity of said slow-wave structure and said predetermined velocity are produced.

2. The microwave oscillator as defined in claim l wherein said means at said one end of said slow-wave structure for producing an electron stream includes a cathode, said cathode being imbedded in said mean-s disposed at said one end of said slow-wave structure for reversing the direction of flow of said electron stream again.

3. A microwave oscillator comprising a slow-wave structure having a co-nstant periodicity and capable of propagating electromagnetic waves along a path at velocities substantially less than the velocity of light; means including a cathode operative to repel electrons and disposed at one end` of said slow-wave structure for producing an electron stream; means for directing said electron stream at a predetermined velocity along said path through said slow-wave structure commencing at said one end, whereby perturbations of the electrons of said stream induce concomitant electromagnetic waves on said slow-wave structure which in turn modulate said electron stream; means disposed at the other end of said slow-wave structure for reversing the direction of flow of said modulated electro-n stream and for directing it slightly radially displaced towards said `cathode at said one end of said slow-wave structure at said predetermined velocity, whereby the modulations of said electron stream induce concomitant electromagnetic waves which in turn increase the magnitude of said modulations; means for providing a reectionless termination at said one end of said slow-wave structure, whereby the electromagnetic waves incident thereon are absorbed, the direction of ow of said electron stream having modulations of increased amplitude being again reversed by said cathode and directed towards said other end of said slow-Wave structure, thereby to produce microwave oscillations having a frequency dependent upon said periodicity and said predetermined velocity.

4. A microwave oscillator comprising a Vslow-wave structure having a constant periodicity and capable of propagating electromagnetic waves at velocities substantially less than the velocity of light along a path; means including a cathode and disposed at one end of said slow-wave structure for producing a hollow cylindrical `and for simultaneously accelerating the electrons of said stream radially outwards, whereby the direction of ow of said electron stream, upon reaching said one end, is reversed by being repelled by said cathode and directed towards said other endof said slow-wave structure in a phase to reinforce the modulations of the electron stream traversing along said path for the first time, thereby to produce microwave oscillations having a frequency dependent upon said periodicity and said predetermined velocity; and means disposed concentrically about said path at said other end of said slow-wave structure for intercepting and collecting said electron stream reversed by said cathode.

5. A microwave oscillator comprising a rectangular waveguide having periodically spaced transverse slots across the broad sides thereof, said waveguide being capable of propagating electromagnetic waves at velocities substantially less than the velocity of light; means including a cathode operative to repel electrons and disposed at one end of said rectangular waveguide for producing an electron stream; means for directing said electron stream at a predetermined velocity along said path through said rectangular waveguide commencing at said one end, whereby perturbations of the electrons of stream induce concomitant electromagnetic waves on said rectangular waveguide which in turn modulate said electron stream; means disposed at the other end of said rectangular waveguide for reversing the direction of ilow of said modulated electron stream and for directing it slightly radially displaced towards said cathode at said one end of said rectangular waveguide at said predetermined velocity; whereby the modulations of said electron stream induce concomitant electromagnetic waves which in turn increase the magnitude of said modulations; means for providing a reflectionless termination at said one end of said rectangular waveguide, whereby the electromagnetic waves incident thereon are absorbed and the direction of flow of said electron stream having modulations of increased amplitude is again reversed by being repelled by said cathode and directed towards said other end of said rectangular waveguide, thereby to produce microwave oscillations having a frequency primarily dependent upon said periodicity and said predetermined velocity and means at said other end of said rectangular waveguide disposed about the path of said electron stream for intercepting and collecting electrons passing along said stream towards said other end for the second time.

References Cited in the tile of this patent UNITED STATES PATENTS 2,337,214 Tuniek Dec. 21, 194s 2,653,270 Kompfner Sept. 22, 1953 2,683,238 Millman July 6, 1954 FOREIGN PATENTS 699,893 Great Britain 2 Nov. 18, 1953 1,080,027 France May 26, 1954 UNITEDl STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No.. 2,869,019 January 13, 1959 Dean Allen Watkins It is herab;T certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line l5,A for Hthe electronically" read to electronically me,"

column H line 6, for Hpassing hroughn read passing through me.

Signed and sealed this 19th day of May 1959.

(SEAL Attest:

ROBERT C. WATSON Commissioner of Patents KAEL H. MINE Attesting @rmer 

